1. Field of the Invention
The invention relates to transparent polymer blends which comprise alkoxysilane-terminated polymers and silicas with a large BET surface area and to a catalyst system which is free from amine functionality.
2. Description of the Related Art
Polymer systems which possess reactive alkoxysilyl groups are well established. In the presence of atmospheric moisture these alkoxysilane-terminated polymers are capable even at room temperature of condensing with one another, with elimination of the alkoxy groups. Depending on the amount of alkoxysilane groups and their construction, the products are principally long-chain polymers (thermoplastics), relatively wide-meshed three-dimensional networks (elastomers) or else highly crosslinked systems (thermosets).
The alkoxysilane-terminated polymers in question may be polymers with an organic backbone, such as polyurethanes, polyesters, polyethers, etc., described inter alia in EP-A-269 819, EP-A-931 800, WO 00/37533, U.S. Pat. No. 3,971,751, and DE 198 49 817, or polymers whose backbone is composed wholly or at least partly of organosiloxanes, described inter alia in WO 96/34030 and U.S. Pat. No. 5,254,657.
In line with the countless possibilities for the design of silane-terminated polymer systems of this kind, not only the properties of the uncrosslinked polymers or of the polymer-containing mixtures (viscosity, melting point, solubilities, etc.) but also the properties of the fully crosslinked compositions (hardness, elasticity, tensile strength, elongation at break, heat resistance, etc.) can be tailored on a virtually custom basis. Correspondingly diverse, therefore, are the possibilities for use of silane-terminated polymer systems of this kind. Thus they can be used, for example, to produce elastomers, sealants, adhesives, elastic adhesive systems, rigid and flexible foams, any of a very wide variety of coating systems, or for impression compounds. These products can be applied in any form, as for example by spreading, spraying, pouring, pressing, knifing, etc., depending on the composition of the formulations.
One particularly interesting application in the adhesives and sealants segment are formulations which remain transparent after curing. Applications for these adhesives and sealants are indicated in those cases where the adhesive seam is not to be visible or where the coloring of the components to be bonded does not allow a suitable selection of colored, filled adhesives. A further advantage of the transparent compositions is that they can be employed more universally. In the case of colored compositions there is always a need to offer a broad range of colors in order to cover the widest possible variety of applications. A particularly high optical challenge in relation to the adhesive bond is posed, for example, in the glazing sector.
Transparent compositions ought also to remain transparent. To date, a large number of the products have exhibited distinct subsequent yellowing during storage and exposure to light, particularly under UV exposure. The reasons for this yellowing are primarily the presence of amine compounds in the formulations. In the case of the typical formulations of silylated polymers, amines are added, mostly in the form of aminosilanes. These compounds serve as adhesion promoters and also as a co-catalyst for curing. Examples of such compositions are described in EP 1041119 A.
In compositions that are filled with chalk, the yellowing can in many cases be compensated by the formulation or simply does not occur. In transparent compositions, there is hardly any possibility of masking yellowing. The addition of dyes—usually bluish dyes—for the purpose of superimposition on the yellowing is common practice. However, it is effective only with limitations, since more severe yellowing produces a “green tint”, which can even turn to brown.